1. Field of the Invention
This invention relates to crystal growth and, in particular, hydrothermal crystal growth.
2. Art Background
The hydrothermal process is utilized for growing a wide variety of crystals. In this process typically a pressure vessel, e.g., a cylindrical pressure vessel, having a baffle or other means to produce a temperature gradient is partially filled at room temperature with an aqueous solution--a growth medium. Crystal nutrient material is introduced on one side of the baffle (or gradient) and seed crystals are introduced on the other. The pressure vessel is sealed and heating is initiated. (This heating is introduced on the nutrient end when the solubility of this nutrient increases with increasing temperature and on the crystal end when solubility decreases with increase in temperature.) As the temperature rises, the growth medium expands, fills the vessel, and produces a significantly elevated internal pressure.
These elevated pressures, e.g., above 15,000 psi, are advantageously utilized in the growth of silicon containing crystals such as quartz crystals. Silicon in the growth medium reacts with the steel pressure vessel to form an iron silicate compound. This silicate coating is resistant to the typically corrosive growth medium, and thus the integrity of the vessel walls are maintained even at high pressures. Nevertheless, high pressures are unacceptable for the growth of essentially silicon-free crystalline compositions, e.g., compositions such as aluminum phosphate, sapphire, ruby, yttrium iron garnet, yttrium aluminum garnet, and II-VI compounds such as zinc oxide, zinc sulfide, or zinc selenide, and other materials having less than 5 mole percent silicon content. If silicon is not present in the growth medium, the vessel walls are not protected and do undergo corrosion. Thus, a pressure vessel with corrosion resistant internal surfaces (surfaces containing noble metals, gold, or silver) is required for growth of silicon-free crystals--crystals that are significant for jewelry and electronic device applications. However, high pressure vessels having only noble metal surfaces in contact with the growth medium and having cross-sectional dimensions large enough (larger than 3/4 inch) to accommodate the expeditious growth of reasonably sized crystals have not been developed. Thus, for the growth of essentially silicon-free crystals, nominal pressures, i.e., pressures below 15,000 psi, are necessitated by the design limitations of the vessel.
To ensure that nominal pressures are encountered during the growth of substantially silicon-free crystals, compositions such as hydroxides, fluorides, phosphates, borates, and chlorides (typically denominated mineralizers) are added to the growth medium. These mineralizers have the effect of decreasing the pressure encountered at a given temperature while increasing the solubility of the nutrient in the growth medium. Despite the reductions in pressures achieved through addition of mineralizers, for obvious safety reasons, care must be taken to maintain relatively low pressures. If an excessive volume of growth medium is initially present in the pressure vessel, as the growth temperature is approached, the pressure increases not only to the level attained when the medium expands to fill the vessel, but thereafter rapidly accelerates to dangerous levels. Pressure vessel temperature curves for various fill levels are not available for medium containing mineralizers but are available for pure water. (See, G. C. Kennedy, American Journal of Science, 248, p. 540 (1950).) Unsafe conditions are generally avoided by initially filling the vessel to a level that is only slightly greater, i.e., less than 5 percent greater, than that yielding acceptable pressures in the absence of a mineralizer.